Polyoxymethylene (POM) resin has been employed as a molding material in the manufacture of various parts such as gears, reels, cord clips, etc. but because these moldings are not good enough in impact strength, many attempts have been made to improve POM resin in this quality parameter.
However, because of the very structure of POM resin, no blending resin is available that is sufficiently compatible with POM resin.
Furthermore, because of the high crystallinity of POM resin, any improvement in its physical properties that may be obtained by alloying with other resins compromises its weld strength and elongation.
Moreover, because of its inadequate thermal stability, POM resin is not suited for high-temperature blending.
Heretofore a number of core-shell polymers have been proposed for melt-blending for the purpose of improving the impact strength of matrix resins. Any core-shell polymer consisting of a rubbery elastomer core and a glassy polymer shell, in particular, has the advantage of greater reproducibility of dispersion uniformity because the state of its dispersion in a matrix resin is less susceptible to the influence of melt-blending conditions.
Such core-shell polymers have heretofore been used as impact modifier for a variety of matrix resins such as polycarbonate, poly(butylene terephthalate), polyamide, poly(phenylene oxide), etc. as well as various alloys thereof.
However, the core-shell polymers heretofore available contain ingredients that encourage thermal degradation of POM resin. Therefore, these known core-shell polymers can hardly be even blended with POM resin. If they could be blended, the resulting compositions would be inadequate in thermal stability.
A POM resin composition with improved impact strength is disclosed in U.S. Pat. No. 4,804,716, for instance. This is a POM resin composition forming thermoplastic IPN (interpenetrated polymer networks) with a polyurethane elastomer but has many disadvantages. Thus, in order to obtain a sufficiently high impact strength, it is necessary to use the polyurethane elastomer in a fairly large proportion so that the modulus of elasticity is markedly sacrificed. Moreover, it is impossible to obtain a composition having satisfactory thermal stability, weatherability, fluidity and weld strength and elongation characteristics.
European Patent Laid-open Publication No. 115,373 discloses a POM resin composition containing a rubbery elastomer prepared by emulsion-polymerization of C.sub.1-8 alkyl acrylates. However, the production of this composition requires special blending conditions and if the ordinary blending conditions are used, a sufficiently stable POM resin composition cannot be obtained. Moreover, no ingenuity has been exercised in regard to thermal stability in the emulsion polymerization stage.
U.S. Pat. No. 4,713,414 discloses a POM resin composition containing a core-shell polymer and a reactive titanate. However, even with this core-shell polymer, the POM resin composition is unstable, undergoing decomposition.
Particularly the core-shell polymer used in the examples described in U.S. Pat. No. 4,713,414 and EP-A-115,373 is deficient in thermal stability (Comparative Example 1 of this specification).
Disclosed in U.S. Pat. No. 4,639,488 is a POM resin composition containing a rubbery elastomer obtained by emulsion polymerization of butadiene but here is no exercise of ingenuity in the emulsion polymer, either, and the thermal stability of this composition is poor.
U.S. Pat. No. 3,749,755 discloses a POM resin composition containing a rubbery elastomer but its thermal stability is unsatisfactory.
Japanese Patent Examined No. 15331/1984 discloses a method for producing a thermoplastic resin like acrylonitrile-acrylate-styrene (AAS resin) using emulsion polymerization technique improved on impact strength. This is, however, copolymer not a blend mixture.
It is generally acknowledged that a polymer blend composed of crystalline polymers is insufficient in the strength and elongation of welds. For example, a POM resin composition containing a poly urethane elastomer as a blending resin for improved impact strength is markedly compromised in weld strength and elongation.
Moreover, among engineering plastics, POM resin does not necessarily rank high in weatherability. When blended with a poly urethane elastomer for improved impact strength, POM resin provides only a composition markedly compromised in weatherability.
In the above state of the art, therefore, development of an impact modifier which, in a POM resin composition, provides sufficient impact strength and insures sufficient weld strength and elongation as well as improved thermal stability has been keenly demanded.
Moreover, POM resin is particularly poor in weatherability among various engineering plastics. This parameter has not been overtly improved by the prior art mentioned above and, therefore, development of a POM resin composition improved not only in impact strength but also in weatherability has been demanded.
The inventors of the present invention explored this field of art to develop a core-shell polymer capable of providing an improved POM resin composition and found that the surfactant and polymerization initiator used in the preparation of the core-shell polymer has had adverse effects on the thermal stability of POM resin. Based on this finding, attempts were made to improve the weld strength and elongation and weatherability of POM resin, and it was ultimately discovered that the above-mentioned problems can be solved all at once by melt-blending a core-shell polymer of the construction described hereinafter. The present invention is predicated on the above findings.